Skid to Stop Formula

SKID TO STOP FORMULA DERIVATION

The following is one method of deriving the formula. It is based on Newtonian Physics and does not involve any empirical coefficients or other experimentally derived values. There are other ways to derive the formula but they all end up at the same place.

1. Equate the initial kinetic energy of the moving object with the work that would have to be done to stop it. This must be equal to satisfy the laws of thermodynamics.

Ke = Work ….1

Ke = ½ x m x v2 ….2

Work = F x d ….3

½ x m x v2 = F x d ….4

This assumes that the object is skidding to a stop with no impacts or changes in surface along the way.

2. Substitute the relationship for the coefficient of friction in place of the stopping force in equation 4 above.

Remember that F = f x w from the ratio f = F / w

 ½ x m x v2 = f x w d ….5

3. Substitute the relationship between weight and mass in place of the weight on the right side of equation 5. (w = m x g)

 ½ x m x v2 = f x m x g x d ….6

4. Cancel out the variables that appear on both sides of the equal sign and re-arrange the variables to a more usable form.

 ½ x v2 = f x g x d ….7

Senior Constable John Twelves, B.Sc., Technical Traffic Collision Investigator Western Region Traffic Unit, Ontario Provincial Police v2 = 2 x f x g d

v = 2 x f x g x d ….8

5. Combine constants (2, gravity) and build in a conversion factor to change the result from m/s to km/hr.

v = 2 x 9.81 x f x d x 3.6 (converts m/s to km/hr)

v = (2 x 3.6) x (9.81 x 3.6) x f x d

The final formula is

S = 254 x f x d

S = 15.9 f e x d

The coefficient f must have the elevation (e) of the road subtracted if the skid is downhill and added if the skid is uphill.

Elevation (e) = rise/run as a decimal.

This formula is independent of the mass of the vehicle, type of tires, inflation pressures and tread pattern. All four tires must be skidding.

Senior Constable John Twelves, B.Sc., Technical Traffic Collision Investigator Western Region Traffic Unit, Ontario Provincial Police